Optical wavelength converting apparatus

ABSTRACT

In an optical wavelength converting apparatus, a fundamental wave impinges upon a crystal of a nonlinear optical material, the type II of phase matching between the fundamental wave and its second harmonic is effected, and the second harmonic of the fundamental wave is thereby radiated out of the optical wavelength converting apparatus. Two crystals constituted of the same material are employed as the crystal. The two crystals have equal lengths and are located in orientations such that corresponding optic axes may be shifted 90° from each other. The optical wavelength converting apparatus yields the second harmonic having the maximum possible output power and yet can be kept small in size and low in cost.

This is a divisional of application Ser. No. 07/843,719, filed Feb. 28,1992, U.S. Pat. No. 5,315,433.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an optical wavelength converting apparatus forconverting a fundamental wave into its second harmonic. This inventionparticularly relates to an optical wavelength converting apparatus,wherein a crystal of a nonlinear optical material, with which the typeII of phase matching between a fundamental wave and its second harmonicis effected, is utilized.

2. Description of the Prior Art

Various attempts have heretofore been made to convert the fundamentalwave of a laser beam into its second harmonic, e.g. to shorten thewavelength of a laser beam, by using a nonlinear optical material. As anoptical wavelength converting apparatus for carrying out such wavelengthconversion, there has heretofore been known a bulk crystal type ofoptical wavelength converting apparatus as described in, for example,"Hikari Electronics No Kiso" (Fundamentals of Optoelectronics) by A.Yariv, translated by Kunio Tada and Takeshi Kamiya, Maruzen K.K., pp.200-204.

Also, laser diode pumped solid lasers have been proposed in, forexample, Japanese Unexamined Patent Publication No. 62(1987)-189783. Theproposed laser diode pumped solid lasers comprise a solid laser rod,which has been doped with a rare earth metal, such as neodymium (Nd).The solid laser rod is pumped by a semiconductor laser (a laser diode).In the laser diode pumped solid laser of this type, in order for a laserbeam having as short a wavelength as possible to be obtained, a bulksingle crystal of a nonlinear optical material for converting thewavelength of a laser beam, which has been produced by solid laseroscillation, is located in a resonator of the solid laser. The laserbeam, which has been produced by the solid laser oscillation, is therebyconverted into its second harmonic, or the like.

As the crystal of the nonlinear optical material, a biaxial crystal,such as a KTP crystal, is often employed. How to effect the phasematching with a KTP biaxial crystal is described in detail by Yao, etal. in J. Appl. Phys., Vol. 55, p. 65, 1984. The method for effectingthe phase matching with a biaxial crystal will be described hereinbelow.

With reference to FIG. 4, the direction, along which a fundamental wavetravels, and the optic axis Z of the crystal make an angle θ. Theprojection of the direction, along which the fundamental wave travelsonto the plane in which the optic axes X and Y lie, and the optic axis Xmake an angle φ. The refractive index of the crystal with respect to thefundamental wave, which impinges upon the crystal at an arbitrary angleof incidence, and the refractive index of the crystal with respect tothe second harmonic of the fundamental wave are represented respectivelyby

    nω, n.sup.2 ω                                  (1)

The refractive indexes of the crystal with respect to the lightcomponents of the fundamental wave, which have been polarizedrespectively in the X, Y, and Z optic axis directions, and therefractive indexes of the crystal with respect to the light componentsof the second harmonic, which have been polarized respectively in the X,Y, and Z optic axis directions, are represented by

    n.sub.X.sup.ω,n.sub.Y.sup.ω,n.sub.Z.sup.ω,n.sub.X.sup.2.omega.,n.sub.Y.sup.2ω,n.sub.Z.sup.2ω            ( 2)

When k_(X), k_(Y), and k_(Z), are defined as follows:

k_(X) =sin θ·cos φ

k_(Y) =sin θ·sin φ

k_(Z) =cos θ

the following formulas obtain: ##EQU1##

Solutions of Formulas (3) and (4) represent the conditions under whichthe phase matching can be effected.

When B1, C1, B2, and C2 are defined as follows: ##EQU2## the solutionsof Formulas (3) and (4) are represented by the formulas ##EQU3## (Doublesigns: + when i=1, and - when i=2)

When the condition

    n.sup.ω,.sub.2 n.sup.2ω,.sub.1                 ( 8)

is satisfied, the phase matching between the fundamental wave and itssecond harmonic is effected. Such phase matching is referred to as thetype I of phase matching.

Also, when the condition

    1/2(n.sup.ω,.sub.1 +n.sup.ω,.sub.2)=n.sup.2ω,.sub.1( 9)

is satisfied, the phase matching between the fundamental wave and itssecond harmonic is effected. Such phase matching is referred to as thetype II of phase matching.

In cases where the type II of phase matching is effected with a biaxialcrystal, the fundamental wave impinging upon the crystal is subjected totwo refractive indexes of the crystal. By way of example, the nonlinearoptical constant d24 of the crystal may be utilized. Specifically, asillustrated in FIG. 5, a fundamental wave 11, which has been polarizedlinearly in the direction indicated by the double headed arrow P, may beintroduced into a crystal 10. The direction indicated by the doubleheaded arrow P inclines at an angle of 45° from the Y optic axis of thecrystal 10 towards the Z axis of the crystal 10. (The fundamental wave11 comprises the linearly polarized light component in the Y axisdirection and the linearly polarized light component in the Z axisdirection.) In this manner, a second harmonic 12, which has beenpolarized linearly in the Y axis direction, may be obtained from thecrystal 10. In such cases, the linearly polarized light component of thefundamental wave 11 in the Z axis direction is subjected to a refractiveindex

    n.sup.ω,.sub.1                                       ( 10)

Also, the linearly polarized light component of the fundamental wave 11in the Y' direction, which direction is normal to the direction oftravel of the fundamental wave 11 and to the Z axis, is subjected to arefractive index

    n.sup.ω,.sub.2                                       ( 11)

Thus the fundamental wave 11 is subjected to the two refractive indexes.

Strictly speaking, in cases where the crystal 10 has been cut into theshape shown in FIG. 5, the fundamental wave 11 impinges upon the crystal10 such that it has been polarized linearly in the Y' direction (whichinclines from the Y axis towards the X axis) and in the Z axisdirection. The second harmonic 12 is obtained as light which has beenpolarized in the Y' direction. However, practically, no problem occurswhen consideration is made in the manner described above.

As described above, in cases where the type II of phase matching is tobe effected with a biaxial crystal, a fundamental wave, which has beenpolarized linearly in one direction, has heretofore impinged upon anonlinear optical material such that polarized light components mayoccur in directions along which the two crystallographic axes of thenonlinear optical material extend. Therefore, the fundamental wave hasheretofore been subjected to two refractive indexes. If the fundamentalwave is subjected to two refractive indexes, a phase difference Δ willoccur between the polarized light components, which are subjected todifferent refractive indexes. The phase difference Δ is represented bythe formula

    Δ=(n.sup.ω,.sub.2 -n.sup.ω,.sub.1)L·2π/λ           (12)

where λ represents the wavelength of the fundamental wave, and Lrepresents the length of the crystal. The length, L, of the crystal isthe effective length, i.e., the length of the optical path of thefundamental wave in the crystal.

If the phase difference Δ occurs, the direction of linear polarizationof the fundamental wave will change in accordance with the value of thephase difference Δ. If the direction of linear polarization of thefundamental wave thus changes, the angles of the direction of linearpolarization of the fundamental wave, with respect to the optic axes ofthe crystal of the nonlinear optical material, will shift from thepredetermined values of the angles, at which the maximum possibleefficiency of wavelength conversion can be achieved. As a result, theoutput power of the second harmonic becomes low. Such fluctuations inthe output power of the second harmonic occur periodically. Thefluctuations in the output power of the second harmonic are classifiedinto those which are dependent on the temperature and which occur in thepattern shown in FIG. 6, in accordance with the dependency of theparameters in Formula (12) upon the temperature, and those which aredependent on the length of the crystal and which occur in the patternshown in FIG. 7.

Therefore, in order for the second harmonic having the maximum possibleoutput power to be obtained, it is necessary that the temperature of thecrystal or the length of the crystal be set to appropriate values. Anexample of an optical wavelength converting apparatus, in which thetemperature of a crystal is adjusted, is disclosed in, for example, U.S.Pat. No. 4,913,533. Also, examples of optical wavelength convertingapparatuses, in which the length of a crystal is adjusted, are disclosedin, for example, Japanese Unexamined Patent Publication Nos.1(1989)-152781 and 1(1989)-152782.

Also, in Japanese Unexamined Patent Publication No. 1(1989)-152781, anoptical wavelength converting apparatus is disclosed in which a crystalof a nonlinear optical material having a trapezoidal cross-sectionalshape is located in a resonator. The crystal is moved up and down withrespect to the trapezoid, and the length of the optical path of thefundamental wave in the crystal is thereby changed. In this manner, thephase difference Δ is adjusted.

However, in cases where the length of the crystal is kept the same, andthe temperature of the crystal is adjusted such that the second harmonichaving the maximum possible output power may be obtained, a largeelectric power source for the adjustment of the temperature and a largeheat sink must be employed such that the temperature of the crystal maybe adjusted to a value falling within a wide range. Therefore, the sizeof the optical wavelength converting apparatus cannot be kept small insize, and the cost of the optical wavelength converting apparatus cannotbe kept low.

Also, ordinarily, in cases where the temperature of the crystal isadjusted to a predetermined appropriate value, it inevitably occurs thatthe position, at which the temperature of the crystal is monitored, (orthe position at which the temperature in the resonator is monitored) andthe part of the crystal, through which the fundamental wave actuallypasses, are spaced apart from each other. Therefore, if the temperaturein the optical wavelength converting apparatus is changed by a change inthe ambient temperature, or the like, the detected temperature will notcoincide with the temperature of the part of the crystal, through whichthe fundamental wave passes. As a result, the temperature of the part ofthe crystal through which the fundamental wave passes will be adjustedto a value different from the desired value. Accordingly, the secondharmonic having the maximum possible output power cannot be obtained.

In cases where the temperature of the crystal is kept the same, and thelength of the crystal is adjusted to a value appropriate for thetemperature of the crystal, the length of the crystal must be adjustedvery strictly. Therefore, it is difficult for the second harmonic havingthe maximum possible output power to be obtained. Even if it is possiblefor the second harmonic having the maximum possible output power to beobtained, because the length of the crystal must be measured andadjusted strictly, the cost of the optical wavelength convertingapparatus cannot be kept low.

With the optical wavelength converting apparatus disclosed in JapaneseUnexamined Patent Publication No. 1(1989)-152781, it is necessary forthe crystal of the nonlinear optical material to be moved acomparatively large distance. Therefore, if the direction of linearpolarization of the fundamental wave is adjusted, the position of theresonator mode will shift from the correct position. Also, with theoptical wavelength converting apparatus disclosed in Japanese UnexaminedPatent Publication No. 1(1989)-152781, the fundamental wave easilyundergoes the longitudinal multimode. As a result, the problem occurs inthat mode competition noise occurs easily.

SUMMARY OF THE INVENTION

The primary object of the present invention is to provide an opticalwavelength converting apparatus, wherein a crystal of a nonlinearoptical material, with which the type II of phase matching between afundamental wave and its second harmonic is effected, is utilized suchthat the second harmonic having the maximum possible output power may beobtained and the optical wavelength converting apparatus may be small insize and low in cost.

Another object of the present invention is to provide an opticalwavelength converting apparatus, with which the aforesaid primary objectis accomplished, and the position of the resonator mode does not shiftfrom the correct position.

The specific object of the present invention is to provide an opticalwavelength converting apparatus, with which the aforesaid primary objectis accomplished, and the second harmonic, having high output power andfree of any mode competition noise, is obtained.

The present invention provides a first optical wavelength convertingapparatus in which a fundamental wave impinges upon a crystal of anonlinear optical material, the type II of phase matching between thefundamental wave and its second harmonic is effected, and the secondharmonic of the fundamental wave is thereby radiated out of the opticalwavelength converting apparatus,

wherein two crystals constituted of the same material are employed assaid crystal, the two said crystals having equal lengths and beinglocated in orientations such that corresponding optic axes may beshifted 90° from each other.

With the first optical wavelength converting apparatus in accordancewith the present invention, by way of example, the two crystals arelocated in orientations shown in FIG. 1. Specifically, a crystal 10 anda crystal 10', which have equal lengths L, are located such that, forexample, the Z axes may be shifted 90° from each other. In such cases, aphase difference Δ with the crystal 10 is represented by the formula

    Δ=(n.sup.107 ,.sub.2 -n.sup.ω,.sub.1)L·2π/λ

Also, a phase difference Δ' with the crystal 10' is represented by theformula

    Δ'=(n.sup.ω,.sub.1 -n.sup.ω,.sub.2)L·2π/λ

Therefore, the phase difference with the crystals 10 and 10' becomeszero (i.e. Δ+Δ'=0).

Even if the length L of the crystal 10 and the length L of the crystal10' are different slightly, the phase difference (Δ+Δ') will be close tozero. Therefore, the period, with which fluctuations in the output powerof the second harmonic due to temperature occur, becomes very long.Accordingly, even if the length L of the crystal 10 and the length L ofthe crystal 10' are different slightly, the second harmonic, havingoutput power close to the maximum value, can be obtained.

As described above, the first optical wavelength converting apparatus inaccordance with the present invention has a simple structure composed oftwo crystals of the nonlinear optical material and can yield the secondharmonic having the maximum intensity. The first optical wavelengthconverting apparatus in accordance with the present invention need notbe provided with a temperature adjusting means, which is large in sizeand should have a high accuracy. Therefore, the first optical wavelengthconverting apparatus in accordance with the present invention can bekept small in size and cheap in cost. Also, with the first opticalwavelength converting apparatus in accordance with the presentinvention, the two crystals of the nonlinear optical material may beprepared such that their lengths are equal to each other. The lengths ofthe two crystals of the nonlinear optical material need not be setstrictly at specific values. Therefore, accurate measurement andadjustment of the lengths of the crystals need not be carried out. Withthis feature, the cost of the first optical wavelength convertingapparatus in accordance with the present invention can be lowered evenfurther.

The present invention also provides a second optical wavelengthconverting apparatus in which a fundamental wave impinges upon a crystalof a nonlinear optical material, the type II of phase matching betweenthe fundamental wave and its second harmonic is effected, and the secondharmonic of the fundamental wave is thereby radiated out of the opticalwavelength converting apparatus,

wherein said crystal is composed of a crystal of a material, therefractive index of which changes with a stress, and

a means applies a stress to said crystal.

In the second optical wavelength converting apparatus in accordance withthe present invention, the means for applying a stress to the crystalshould preferably apply the stress in a single optic axis direction,which intersects perpendicularly to the direction of incidence of thefundamental wave upon the crystal.

Effects of the application of a stress will be described hereinbelow bytaking the configuration shown in FIG. 5 as an example. By way ofexample, a stress is applied to the crystal 10 only in the Z axisdirection. When a stress is applied in the Z axis direction, therefractive index

    n.sup.ω,.sub.1

i.e. the refractive index to which the linearly polarized lightcomponent of the fundamental wave 11 in the Z axis direction issubjected, changes. As described above, the phase difference Δ with thecrystal 10 is represented by the formula

    Δ=(n.sup.ω,.sub.2 -n.sup.ω,.sub.1)L·2π/λ

Therefore, by changing the refractive index

    n.sup.ω,.sub.1

the phase difference Δ can be made equal to or approximately equal to2nπ, where n represents an integer. If the phase difference Δ is closeto 2nπ, the period, with which fluctuations in the output power of thesecond harmonic due to temperature occur, becomes very long.Accordingly, even if the phase difference Δ is not equal to 2nπ, but isclose to 2nπ, the second harmonic, having output power close to themaximum value, can be obtained.

The means for applying a stress to the crystal may apply the stress in adirection shifted from the single optic axis direction, which intersectsperpendicularly to the direction of incidence of the fundamental waveupon the crystal. Specifically, by way of example, in FIG. 5, the meansfor applying a stress to the crystal may apply a stress, which has acomponent in the Z axis direction and a component in the Y' axisdirection. Even in such cases, a much larger stress can be applied inone optic axis direction than in the other optic axis direction,depending on the direction in which the stress is applied. Also, incases where an equal stress is applied in the two optic axis directions,if the change characteristics of the refractive index

    n.sup.ω,.sub.1

and the refractive index

    n.sup.ω,.sub.2

with respect to the stress are different from each other, the phasedifference Δ can be made equal to 2nπ, where n represents an integer.

Also, the means for applying a stress to the crystal may apply apositive stress or a negative stress. Alternatively, the means forapplying a stress to the crystal may apply a positive stress in onedirection and a negative stress in the other direction.

As described above, the second optical wavelength converting apparatusin accordance with the present invention has a simple structure composedof the means for applying a stress to the crystal of the nonlinearoptical material and can yield the second harmonic having the maximumintensity. The second optical wavelength converting apparatus inaccordance with the present invention need not be provided with atemperature adjusting means, which is large in size and should have ahigh accuracy. Therefore, the second optical wavelength convertingapparatus in accordance with the present invention can be kept small insize and cheap in cost. Also, with the second optical wavelengthconverting apparatus in accordance with the present invention, thelength of the crystal of the nonlinear optical material need not be setstrictly at a specific value. Therefore, accurate measurement andadjustment of the length of the crystal need not be carried out. Withthis feature, the cost of the second optical wavelength convertingapparatus in accordance with the present invention can be lowered evenfurther.

The present invention further provides a third optical wavelengthconverting apparatus in which a fundamental wave impinges upon a crystalof a nonlinear optical material, the type II of phase matching betweenthe fundamental wave and its second harmonic is effected, and the secondharmonic of the fundamental wave is thereby radiated out of the opticalwavelength converting apparatus,

wherein the improvement comprises the provision of:

i) a means for adjusting a difference in phase of said fundamental wavedue to said crystal,

ii) a means for separating said fundamental wave into two polarizedlight components, which intersect perpendicularly to each other,

iii) a means for detecting the light intensity of one of said polarizedlight components, which have been separated from each other,

iv) a means for detecting the light intensity of the other of saidpolarized light components, which have been separated from each other,and

v) a control means for controlling said means for adjusting a differencein phase such that a predetermined relationship may be kept between thelight intensities of the two polarized light components, which lightintensities are detected by two said detection means.

As the means for adjusting a difference in phase, by way of example, ameans for adjusting the temperature of the crystal of the nonlinearoptical material, a means for applying a voltage to the crystal, or ameans for applying a stress to the crystal may be employed.Alternatively, as the means for adjusting a difference in phase, acombination of a phase compensating device, which is inserted into theoptical path of the fundamental wave, and a means for changing theposition, the orientation, or the like, of the phase compensating devicemay be employed.

Effects of the third optical wavelength converting apparatus inaccordance with the present invention will be described hereinbelow withreference to, for example, FIG. 5. If the direction of linearpolarization of the fundamental wave 11 makes an angle of 45° withrespect to the Z axis, the second harmonic 12 having the maximumintensity can be obtained. Therefore, the fundamental wave 11 isseparated into the linearly polarized light component in the Z axisdirection and the linearly polarized light component in the Y' axisdirection. The means for adjusting a difference in phase is thencontrolled such that the light intensity of the linearly polarized lightcomponent in the Z axis direction and the light intensity of thelinearly polarized light component in the Y' axis direction may becomeequal to each other. As a result, the direction of linear polarizationof the fundamental wave 11 makes an angle of 45° with respect to the Zaxis. Therefore, the second harmonic 12, having the maximum intensity,can be obtained.

As described above, with the third optical wavelength convertingapparatus in accordance with the present invention, the actual directionof linear polarization of the fundamental wave is detected, and themeans for adjusting a difference in phase is controlled such that thedirection of linear polarization of the fundamental wave may coincidewith a predetermined direction. Therefore, no problem due to an error inthe monitored temperature occurs, and the second harmonic, having themaximum intensity, can be obtained reliably.

Also, with the third optical wavelength converting apparatus inaccordance with the present invention, accurate measurement andadjustment of the length of the crystal need not be carried out.Therefore, the cost of the third optical wavelength converting apparatusin accordance with the present invention can be kept low.

The present invention still further provides a fourth optical wavelengthconverting apparatus in which a fundamental wave impinges upon a crystalof a nonlinear optical material, the type II of phase matching betweenthe fundamental wave and its second harmonic is effected, and the secondharmonic of the fundamental wave is thereby radiated out of the opticalwavelength converting apparatus,

wherein the improvement comprises the provision of a means for rotatingsaid crystal around an axis, which extends in a direction thatintersects an optical path of said fundamental wave in said crystal, andthereby changing the length of the optical path of said fundamental wavein said crystal.

The length of the optical path of the fundamental wave in the crystalcorresponds to the crystal length L in Formula (12). If the crystallength L changes, the phase difference Δ will change. When the phasedifference Δ changes, the direction of linear polarization of thefundamental wave also changes. Therefore, by rotating the crystal of thenonlinear optical material in the manner described above, the directionof linear polarization of the fundamental wave can be adjusted such thatthe maximum wavelength conversion efficiency may be obtained.

As described above, the fourth optical wavelength converting apparatusin accordance with the present invention has a simple structure composedof the means for rotating the crystal of the nonlinear optical materialand can yield the second harmonic having the maximum intensity. Thefourth optical wavelength converting apparatus in accordance with thepresent invention need not be provided with a temperature adjustingmeans, which is large in size and should have a high accuracy.Therefore, the fourth optical wavelength converting apparatus inaccordance with the present invention can be kept small in size andcheap in cost. Also, with the fourth optical wavelength convertingapparatus in accordance with the present invention, the length of thecrystal of the nonlinear optical material need not be set strictly at aspecific value. Therefore, accurate measurement and adjustment of thelength of the crystal need not be carried out. With this feature, thecost of the fourth optical wavelength converting apparatus in accordancewith the present invention can be lowered even further.

Also, with the fourth optical wavelength converting apparatus inaccordance with the present invention, by selecting an appropriate angleof rotation of the crystal of the nonlinear optical material, theoccurrence of any mode competition noise can be eliminated even if thefundamental wave is being generated in a multimode.

The present invention also provides a fifth optical wavelengthconverting apparatus in which a laser beam, serving as a fundamentalwave and having been obtained by pumping a solid laser medium, impingesupon a crystal of a nonlinear optical material, the type II of phasematching between the fundamental wave and its second harmonic iseffected, and the second harmonic of the fundamental wave is therebyradiated out of the optical wavelength converting apparatus,

wherein two solid laser media, which produce linearly polarized laserbeams, are employed as said solid laser medium,

one of two said solid laser media is located such that the direction oflinear polarization of the laser beam, which has been produced by saidone solid laser medium, may coincide with the direction of one of twooptic axes of said crystal, and

the other solid laser medium is located such that the direction oflinear polarization of the laser beam, which has been produced by saidother solid laser medium, may coincide with the direction of the otheroptic axis of said crystal.

Effects of the fifth optical wavelength converting apparatus inaccordance with the present invention will be described hereinbelow withreference to, for example, FIG. 5. In order for the second harmonic 12to be produced, it is necessary that the fundamental wave 11 has thelinearly polarized light component in the Z axis direction and thelinearly polarized light component in the Y' axis direction. Therefore,in the fifth optical wavelength converting apparatus in accordance withthe present invention, the two solid laser media are located such thatthe directions of linear polarization of the laser beams, which havebeen produced by the two solid laser media coincide with the Z axisdirection and the Y' axis direction. The two laser beams, serving as thefundamental wave, impinge upon the crystal of the nonlinear opticalmaterial. In this manner, the fundamental wave is converted into itssecond harmonic.

One of the two laser beams is subjected only to one of the tworefractive indexes represented by Formulas (10) and (11). The otherlaser beam is subjected only to the other of the two refractive indexesrepresented by Formulas (10) and (11). Therefore, even if the aforesaidphase difference Δ occurs, the direction of linear polarization of thefundamental wave will not fluctuate in accordance with the value of thephase difference Δ.

As described above, the fifth optical wavelength converting apparatus inaccordance with the present invention has a simple structure composed ofthe two solid laser media and can yield the second harmonic, having themaximum intensity, by preventing any phase difference from occurring inthe fundamental wave. The fifth optical wavelength converting apparatusin accordance with the present invention need not be provided with atemperature adjusting means, which is large in size and should have ahigh accuracy. Therefore, the fifth optical wavelength convertingapparatus in accordance with the present invention can be kept small insize and cheap in cost. Also, with the fifth optical wavelengthconverting apparatus in accordance with the present invention, thelength of the crystal of the nonlinear optical material need not be setstrictly at a specific value. Therefore, accurate measurement andadjustment of the length of the crystal need not be carried out. Withthis feature, the cost of the fifth optical wavelength convertingapparatus in accordance with the present invention can be lowered evenfurther.

The present invention further provides a sixth optical wavelengthconverting apparatus in which a laser beam, serving as a fundamentalwave and having been obtained by pumping a solid laser medium, impingesupon a crystal of a nonlinear optical material, the type II of phasematching between the fundamental wave and its second harmonic iseffected, and the second harmonic of the fundamental wave is therebyradiated out of the optical wavelength converting apparatus,

wherein a solid laser medium, which exhibits birefringence and thethickness of which changes gradually along the direction that intersectsthe optical path of said fundamental wave in the solid laser medium, isemployed as said solid laser medium, and

a means which moves said solid laser medium, with respect to a pumpingsource, along said direction that intersects the optical path of saidfundamental wave.

In cases where the solid laser medium, which exhibits birefringence, isemployed, a phase difference Δ' occurs in the solid laser medium in thesame manner as the phase difference Δ, which is represented by Formula(12) and which occurs in the crystal of the nonlinear optical material.At this time, the solid laser medium, the thickness of which changesgradually along the direction that intersects the optical path of thefundamental wave in the solid laser medium, is moved with respect to thepumping source along the direction that intersects the optical path ofthe fundamental wave. As a result, the effective length of the opticalpath in the solid laser medium changes. Therefore, the phase differenceΔ' changes, and the direction of linear polarization of the fundamentalwave changes in accordance with the change in the phase difference Δ'.Accordingly, by appropriately adjusting the distance of the movement ofthe solid laser medium, the direction of linear polarization of thefundamental wave, with respect to the crystal of the nonlinear opticalmaterial, can be set such that the maximum wavelength conversionefficiency may be obtained. In this manner, a laser beam, the wavelengthof which has been converted into a shorter wavelength and which has ahigh intensity, can be obtained.

In general, a solid laser medium exhibits larger birefringence than thecrystal of a nonlinear optical material. Therefore, even if the rate ofthe change in the thickness of the solid laser medium (i.e., the angleof inclination of the edge face of the solid laser medium) iscomparatively small, and even if the distance of the movement of thesolid laser medium is comparatively small, the phase difference Δ' canbe changed significantly. Accordingly, with the sixth optical wavelengthconverting apparatus in accordance with the present invention, thedirection of linear polarization of the fundamental wave can be setappropriately with a small amount of adjustment such that the positionof the resonator mode may not shift.

Also, the sixth optical wavelength converting apparatus in accordancewith the present invention need not be provided with a temperatureadjusting means, which is large in size and should have a high accuracy.Additionally, with the sixth optical wavelength converting apparatus inaccordance with the present invention, accurate measurement andadjustment of the length of the crystal need not be carried out.Therefore, the sixth optical wavelength converting apparatus inaccordance with the present invention can be kept small in size andcheap in cost.

Further, with the sixth optical wavelength converting apparatus inaccordance with the present invention, by adjusting the distance of themovement of the solid laser medium, oscillation of the solid lasermedium can be effected at the position corresponding to an appropriatethickness of the solid laser medium, and the level of birefringence inthe resonator can be adjusted appropriately. Therefore, the occurrenceof any mode competition noise can be eliminated even if the fundamentalwave is being generated in a multimode.

As will be understood from the specification, it should be noted thatthe term "moving a solid laser medium with respect to a pumping source"as used herein means movement of the solid laser medium relative to thepumping source, and embraces both the cases wherein the solid lasermedium is moved while the pumping source is kept stationary, and caseswherein the pumping source is moved while the solid laser medium is keptstationary.

The present invention still further provides a seventh opticalwavelength converting apparatus, which comprises:

i) a crystal of a nonlinear optical material, which is located in aresonator of a laser diode pumped solid laser, said crystal converting alaser beam, which has been obtained from solid laser oscillation andserves as a fundamental wave which impinges upon said crystal, into itssecond harmonic by effecting the type II of phase matching between saidfundamental wave and its second harmonic, and

ii) a wavelength selecting device, which is located in said resonatorand selects the wavelength of said laser beam, having been obtained fromthe solid laser oscillation such that the wavelength of said laser beammay be adjusted appropriately.

With the seventh optical wavelength converting apparatus in accordancewith the present invention, the wavelength of the laser beam, which hasbeen obtained from the solid laser oscillation, is changed.Specifically, the wavelength λ of the fundamental wave in Formula (12)changes. Also, the refractive indexes

    n.sup.ω,.sub.2 and n.sup.ω,.sub.1

change. Therefore, the value of the phase difference Δ changes, and thedirection of linear polarization of the fundamental wave therebychanges. Accordingly, when the wavelength of the laser beam, which hasbeen obtained from the solid laser oscillation, is appropriatelyadjusted by the wavelength selecting device, the direction of linearpolarization of the fundamental wave, with respect to the crystal of thenonlinear optical material, can be set such that the maximum wavelengthconversion efficiency may be obtained. In this manner, a laser beam, thewavelength of which has been converted into a shorter wavelength andwhich has a high intensity, can be obtained.

Also, the seventh optical wavelength converting apparatus in accordancewith the present invention need not be provided with a temperatureadjusting means, which is large in size and should have a high accuracy.Additionally, with the seventh optical wavelength converting apparatusin accordance with the present invention, accurate measurement andadjustment of the length of the crystal need not be carried out.Therefore, the seventh optical wavelength converting apparatus inaccordance with the present invention can be kept small in size andcheap in cost.

Further, with the seventh optical wavelength converting apparatus inaccordance with the present invention, by the effects of the wavelengthselecting device, the solid laser oscillates in the single longitudinalmode. Therefore, no mode competition noise occurs in the laser beam, thewavelength of which has been converted into a shorter wavelength.

The present invention also provides an eighth optical wavelengthconverting apparatus comprising a crystal of a nonlinear opticalmaterial, which is located in a resonator of a laser diode pumped solidlaser, wherein said crystal converts a laser beam, which has beenobtained from solid laser oscillation and serves as a fundamental wavewhich impinges upon said crystal, into its second harmonic by effectingthe type II of phase matching between said fundamental wave and itssecond harmonic,

wherein at least either one of faces of said crystal, through whichfaces said fundamental wave passes, is convex, and

a means which moves said crystal in a direction such that the length ofthe optical path of said fundamental wave in said crystal may change.

The length of the optical path of the fundamental wave in the crystalcorresponds to the crystal length L in Formula (12). If the crystallength L changes, the phase difference Δ will change. When the phasedifference Δ changes, the direction of linear polarization of thefundamental wave also changes. Therefore, by moving the crystal of thenonlinear optical material in the manner described above, the directionof linear polarization of the fundamental wave can be adjusted such thatthe maximum wavelength conversion efficiency may be obtained.

Also, with the eighth optical wavelength converting apparatus inaccordance with the present invention, at least either one of the facesof the crystal, through which faces the fundamental wave passes, isconvex. Therefore, the solid laser easily oscillates in a single mode.Accordingly, mode competition noise can be prevented from occurring, andthe second harmonic free of any noise can be obtained.

Additionally, because the convex face of the crystal has lens effects,the diameter of the laser beam in the crystal of the nonlinear opticalmaterial becomes small. Therefore, the wavelength conversion efficiencycan be kept high.

Further, the eighth optical wavelength converting apparatus inaccordance with the present invention need not be provided with atemperature adjusting means, which is large in size and should have ahigh accuracy. Therefore, the eighth optical wavelength convertingapparatus in accordance with the present invention can be kept small insize and cheap in cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing the locations of two crystals of anonlinear optical material in a first embodiment of the opticalwavelength converting apparatus in accordance with the presentinvention,

FIG. 2 is a side view showing the first embodiment,

FIG. 3 is a side view showing a second embodiment of the opticalwavelength converting apparatus in accordance with the presentinvention,

FIG. 4 is an explanatory view showing an angle θ between the directionof travel of a fundamental wave in a crystal and an optic axis Z, and anangle φ between the direction of travel of the fundamental wave and anoptic axis X,

FIG. 5 is an explanatory view showing periodic fluctuations in theoutput power of a second harmonic,

FIG. 6 is a graph showing periodic fluctuations in the output power of asecond harmonic, which depend on a change in temperature,

FIG. 7 is a graph showing periodic fluctuations in the output power of asecond harmonic, which depend on a crystal length,

FIGS. 8A and 8B are schematic views showing an example of how to makethe crystal of a nonlinear optical material, which is employed in theoptical wavelength converting apparatus in accordance with the presentinvention,

FIGS. 9A and 9B are schematic views showing a different example of howto make the crystal of a nonlinear optical material, which is employedin the optical wavelength converting apparatus in accordance with thepresent invention,

FIG. 10 is a partially cutaway perspective view showing the major partof a third embodiment of the optical wavelength converting apparatus inaccordance with the present invention,

FIG. 11 is a partially cutaway side view showing the third embodiment,

FIG. 12 is a partially cutaway side view showing a fourth embodiment ofthe optical wavelength converting apparatus in accordance with thepresent invention,

FIG. 13 is a side view showing a fifth embodiment of the opticalwavelength converting apparatus in accordance with the presentinvention,

FIG. 14 is a side view showing a sixth embodiment of the opticalwavelength converting apparatus in accordance with the presentinvention,

FIG. 15 is a front view showing an example of a means for adjusting adifference in phase in the optical wavelength converting apparatus inaccordance with the present invention,

FIG. 16 is a schematic side view showing a different example of a meansfor adjusting a difference in phase in the optical wavelength convertingapparatus in accordance with the present invention,

FIG. 17 is a side view showing a seventh embodiment of the opticalwavelength converting apparatus in accordance with the presentinvention,

FIG. 18 is a perspective view showing the major part of the seventhembodiment,

FIG. 19 is a side view showing an eighth embodiment of the opticalwavelength converting apparatus in accordance with the presentinvention,

FIG. 20 is a side view showing a ninth embodiment of the opticalwavelength converting apparatus in accordance with the presentinvention,

FIG. 21 is a perspective view showing the major part of the ninthembodiment,

FIG. 22 is a side view showing a tenth embodiment of the opticalwavelength converting apparatus in accordance with the presentinvention,

FIG. 23 is a side view showing an eleventh embodiment of the opticalwavelength converting apparatus in accordance with the presentinvention,

FIG. 24 is a schematic perspective view showing the major part of theeleventh embodiment,

FIG. 25 is a side view showing a twelfth embodiment of the opticalwavelength converting apparatus in accordance with the presentinvention,

FIG. 26 is a side view showing a thirteenth embodiment of the opticalwavelength converting apparatus in accordance with the presentinvention,

FIG. 27 is a side view showing a fourteenth embodiment of the opticalwavelength converting apparatus in accordance with the presentinvention,

FIG. 28 is a side view showing a fifteenth embodiment of the opticalwavelength converting apparatus in accordance with the presentinvention,

FIG. 29 is a side view showing a sixteenth embodiment of the opticalwavelength converting apparatus in accordance with the presentinvention,

FIG. 30 is a side view showing a seventeenth embodiment of the opticalwavelength converting apparatus in accordance with the presentinvention,

FIG. 31 is a perspective view showing the major part of the seventeenthembodiment, and

FIG. 32 is a side view showing an eighteenth embodiment of the opticalwavelength converting apparatus in accordance with the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will hereinbelow be described in further detailwith reference to the accompanying drawings.

FIG. 2 shows a first embodiment of the optical wavelength convertingapparatus in accordance with the present invention.

By way of example, this embodiment is incorporated in a laser diodepumped solid laser. The laser diode pumped solid laser comprises asemiconductor laser (a phased array laser) 14, which produces a laserbeam 13 serving as a pumping beam, and a condensing lens 15 forcondensing the laser beam 13, which is a divergent beam. The laser diodepumped solid laser also comprises a YVO₄ rod 16, which is a solid laserrod added with neodymium (Nd). (The YVO₄ rod, which is a solid laser rodadded with neodymium, will hereinafter be referred to as the Nd:YVO₄rod.) The laser diode pumped solid laser further comprises a resonator17, which is located on the side downstream from the Nd:YVO₄ rod 16,i.e. on the right side of the Nd:YVO₄ rod 16 in FIG. 2, and KTP crystals10, 10', which are located between the Nd:YVO₄ rod 16 and the resonator17. These elements 10 through 17 are mounted together with one anotheron a common case (not shown). The temperature of the phased array laser14 is set at a predetermined temperature by a Peltier apparatus (notshown) and a temperature adjusting circuit (not shown).

The phased array laser 14 produces the laser beam 13 having a wavelengthλ1 of 809 nm. The neodymium atoms contained in the Nd:YVO₄ rod 16 arestimulated by the laser beam 13, and the Nd:YVO₄ rod 16 produces a laserbeam 11 having a wavelength λ2 of 1,064 nm.

A light input face 16a of the Nd:YVO₄ rod 16 is provided with a coating18, which substantially reflects the laser beam 11 having the wavelengthof 1,064 nm (with a reflectivity of at least 99.9%) and whichsubstantially transmits the pumping laser beam 13 having the wavelengthof 809 nm (with a transmittance of at least 99%). A face 17a of theresonator 17, which face is located on the side of the KTP crystals 10and 10', takes on the form of part of a spherical surface. The face 17aof the resonator 17 is provided with a coating 19, which substantiallyreflects the laser beam 11 having the wavelength of 1,064 nm and thelaser beam 13 having the wavelength of 809 nm and which substantiallytransmits a second harmonic 12 having a wavelength of 532 nm. Therefore,the laser beam 11 having the wavelength of 1,064 nm is confined betweenthe face 16a and the face 17a, and laser oscillation is thereby causedto occur.

The laser beam 11 impinges upon the KTP crystals 10 and 10' of anonlinear optical material, and is converted thereby into the secondharmonic 12 having the wavelength, which is one half of the wavelengthof the laser beam 11, i.e. is equal to 532 nm. Because the face 17a ofthe resonator 17 is provided with the coating 19, approximately only thesecond harmonic 12 is radiated out of the resonator 17.

As illustrated in detail in FIG. 1, the KTP crystal 10, which is abiaxial crystal, has been cut along the plane which has been rotated 24°from the YZ plane around the Z axis. (The KTP crystal 10' has also beencut in the same manner as the KTP crystal 10.) The KTP crystal 10 islocated such that the direction of linear polarization of the laser beam11, which direction is indicated by the double headed arrow P, may makean angle of 45° with respect to the Z axis. The KTP crystal 10' islocated such that its Z axis may be shifted 90° from the Z axis of theKTP crystal 10. Also, the crystal length L of the KTP crystal 10' isequal to the crystal length L of the KTP crystal 10.

By locating the KTP crystals 10 and 10' in the manner described abovewith respect to the laser beam 11, the type II of phase matching betweenthe laser beam 11, which serves as the fundamental wave, and its secondharmonic 12 can be effected. Also, by locating the KTP crystal 10 andthe KTP crystal 10' in the manner described above with respect to eachother, the phase difference Δ and the phase difference Δ' occurring inthe laser beam 11 are canceled with each other. Therefore, for reasonsdescribed above, the second harmonic 12 having the maximum output powercan be obtained.

The KTP crystals 10 and 10', having the exactly equal length L can bemade, for example, in the manner described below. Specifically, asillustrated in FIG. 8A, two crystals 10 and 10' are adhered to eachother. Thereafter, the upper surface 10a and the lower surface 10b ofthe combined body are polished such that they may accurately becomeparallel to each other (e.g. with parallelism error falling within therange of 10 to 20 seconds). The KTP crystals 10 and 10' are thenseparated from each other. Thereafter, as illustrated in FIG. 8B, theKTP crystals 10 and 10' are secured to each other such that two polishedsurfaces may be in close contact with each other.

Alternatively, as illustrated in FIG. 9A, the upper surface 10a and thelower surface 10b of a single KTP crystal 10A are polished such thatthey may become parallel to each other. The KTP crystal 10A is then cutalong a plane 10B, which passes through the polished surfaces 10a and10b, and two KTP crystals 10 and 10' are thereby obtained. Thereafter,as illustrated in FIG. 9B, the KTP crystals 10 and 10' are secured toeach other such that two polished surfaces may be in close contact witheach other.

A second embodiment of the optical wavelength converting apparatus inaccordance with the present invention will be described hereinbelow withreference to FIG. 3.

By way of example, this embodiment is incorporated in a laser diodepumped solid laser. In this embodiment, aNd:YVO₄ rod 16 and KTP crystals10, 10' are secured to a heat sink 20, which may be constituted ofcopper, or the like. The heat sink 20 is secured to a TE cooler 21,which may be constituted of a Peltier apparatus, or the like. The TEcooler 21 is operated by a driving circuit (not shown) in order to keepthe temperatures of the Nd:YVO₄ rod 16 and the KTP crystals 10, 10' atpredetermined values.

A phased array laser 14 is located such that it may be in close contactwith a face 16a of the Nd:YVO₄ rod 16. Also, the phased array laser 14is secured to the heat sink 20 such that it may be in close contact withthe heat sink 20. The phased array laser 14 produces a laser beam 13having a wavelength λ1 of 809 nm. The neodymium atoms contained in theNd:YVO₄ rod 16 are stimulated by the laser beam 13, and the Nd:YVO₄ rod16 produces a laser beam 11 having a wavelength λ2 of 1,064 nm.

A face 16a of the Nd:YVO₄ rod 16 is provided with a coating 22, whichsubstantially reflects the laser beam 11 having the wavelength of 1,064nm and which substantially transmits the pumping laser beam 13 havingthe wavelength of 809 nm. A face 10c of the KTP crystal 10' is providedwith a coating 23, which substantially reflects the laser beam 11 havingthe wavelength of 1,064 nm and which substantially transmits a secondharmonic 12 having a wavelength of 532 nm.

Also, the other face 16b of the Nd:YVO₄ rod 16 is provided with acoating 24, which substantially transmits the laser beam 11 and whichsubstantially reflects the pumping laser beam 13 and the second harmonic12.

Therefore, the laser beam 11 having the wavelength of 1,064 nm isconfined between the face 16a and the face 10c, and laser oscillation isthereby caused to occur. The laser beam 11 impinges upon the KTPcrystals 10 and 10', and is converted thereby into the second harmonic12 having the wavelength λ3 of 532 nm. Because the face 10c of the KTPcrystal 10' is provided with the coating 23, the second harmonic 12 isefficiently radiated out of the KTP crystal 10'.

In this embodiment, the two KTP crystals 10 and 10' have an equal lengthand are located such that the corresponding optic axes are deviated 90°from each other. Therefore, with the second embodiment, as in the firstembodiment, the second harmonic 12 having the maximum output power canbe obtained.

In the first and second embodiments described above, the fundamentalwave, which has been polarized linearly, impinges upon the KTP crystals10 and 10'. However, the optical wavelength converting apparatus inaccordance with the present invention is also applicable when anunpolarized fundamental wave is converted into its second harmonic.Also, in such cases, the same effects as those described above can beobtained with the optical wavelength converting apparatus in accordancewith the present invention. The optical wavelength converting apparatusin accordance with the present invention is additionally applicable whencrystals of a nonlinear optical material other than KTP are employed.

The solid laser medium employed in the optical wavelength convertingapparatus in accordance with the present invention is not limited toNd:YVO₄ and may be selected from any other known media, e.g. a directcompound laser crystal, such as LNP, NAB, or NPP.

Different embodiments of the optical wavelength converting apparatus inaccordance with the present invention will be described hereinbelow. Inthe drawings described below, similar elements are numbered with thesame reference numerals with respect to FIGS. 1, 2, and 3.

FIG. 11 shows a third embodiment of the optical wavelength convertingapparatus in accordance with the present invention.

By way of example, this embodiment is incorporated in a laser diodepumped solid laser. The laser diode pumped solid laser comprises thephased array laser 14, which produces the laser beam 13 serving as apumping beam, and the condensing lens 15. The laser diode pumped solidlaser also comprises the Nd:YVO₄ rod 16 and the KTP crystal 10, which islocated on the side downstream from the Nd:YVO₄ rod 16, i.e. on theright side of the Nd:YVO₄ rod 16 in FIG. 11.

The light input face 16a of the Nd:YVO₄ rod 16 is provided with acoating 118, which is of the same type as the coating 18 in theembodiment of FIG. 2. The light output face 10a of the KTP crystal 10takes on the form of part of a spherical surface. The face 10a of theKTP crystal 10 is provided with a coating 119, which is of the same typeas the coating 19 in the embodiment of FIG. 2. Therefore, the laser beam11 having the wavelength of 1,064 nm is confined between the face 16aand the face 10a, and laser oscillation is thereby caused to occur.

The laser beam 11 impinges upon the KTP crystal 10 of a nonlinearoptical material, and is converted thereby into the second harmonic 12having the wavelength, which is one half of the wavelength of the laserbeam 11, i.e. is equal to 532 nm. Because the face 10a of the KTPcrystal 10 is provided with the coating 119, approximately only thesecond harmonic 12 is radiated out of the KTP crystal 10.

As illustrated in detail in FIG. 10, the KTP crystal 10, which is abiaxial crystal, has been cut along the plane which has been rotated 24°from the YZ plane around the Z axis. The KTP crystal 10 is located suchthat the direction of linear polarization of the laser beam 11, whichdirection is indicated by the double headed arrow P, may make an angleof 45° with respect to the Z axis. By locating the KTP crystal 10 inthis manner, the type II of phase matching between the laser beam 11,which serves as the fundamental wave, and its second harmonic 12 can beeffected.

The KTP crystal 10 is incorporated in a stress application means 130.The stress application means 130 comprises a square frame-like supportblock 131, and a pushing plate 132, which can move vertically in FIGS.10 and 11 inside of the space defined by the support block 131. Thestress application means 130 also comprises a set-screw 133, which isengaged by threads with the upper part of the support block 131 suchthat the leading end of the set-screw 133 may face the pushing plate132. The KTP crystal 10 is accommodated inside of the space defined bythe support block 131 such that the Z axis may extend vertically (i.e.,in the direction along which the set-screw 133 moves forwardly andreversely by threads). The lower face of the KTP crystal 10 is incontact with the bottom surface of the support block 131, and the upperface of the KTP crystal 10 is in contact with the pushing plate 132.

When the set-screw 133 is tightened in this state, a stress in the Zaxis direction is applied to the KTP crystal 10. As the set-screw 133 istightened more strongly, a larger stress is applied to the KTP crystal10. When the stress applied to the KTP crystal 10 is increased little bylittle to a predetermined value, the phase difference Δ, which has beencaused to occur by the KTP crystal 10, becomes eliminated. Therefore,for reasons described above, the second harmonic 12, having the maximumoutput power, can be obtained.

The value of the stress applied to the KTP crystal 10 can thus be set atan appropriate value. Therefore, in cases where the laser beam 11, whichserves as the fundamental wave, is being produced in a multimode, themode competition noise can be prevented from occurring.

A fourth embodiment of the optical wavelength converting apparatus inaccordance with the present invention will be described hereinbelow withreference to FIG. 12.

By way of example, this embodiment is incorporated in a laser diodepumped solid laser. In this embodiment, the Nd:YVO₄ rod 16 and the KTPcrystal 10 are secured to a heat sink 120, which may be constituted ofcopper, or the like. The heat sink 120 is secured to a TE cooler 121,which may be constituted of a Peltier apparatus, or the like. The TEcooler 121 is operated by a driving circuit (not shown) in order to keepthe temperatures of the Nd:YVO₄ rod 16 and the KTP crystal 10 atpredetermined values.

The phased array laser 14 is located such that it may be in closecontact with the face 16a of the Nd:YVO₄ rod 16. Also, the phased arraylaser 14 is secured to the heat sink 120 such that it may be in closecontact with the heat sink 120. The phased array laser 14 produces thelaser beam 13 having a wavelength λ of 809 nm. The neodymium atomscontained in the Nd:YVO₄ rod 16 are stimulated by the laser beam 13, andthe Nd:YVO₄ rod 16 produces the laser beam 11 having a wavelength λ2 of1,064 nm.

The face 16a of the Nd:YVO₄ rod 16 is provided with a coating 122, whichsubstantially reflects the laser beam 11 having the wavelength of 1,064nm and which substantially transmits the pumping laser beam 13 havingthe wavelength of 809 nm. The face 10a of the KTP crystal 10 is providedwith a coating 123, which substantially reflects the laser beam 11having the wavelength of 1,064 nm and which substantially transmits asecond harmonic 12 having a wavelength of 532 nm.

Also, the other face 16b of the Nd:YVO₄ rod 16 is provided with acoating 124, which substantially transmits the laser beam 11 and whichsubstantially reflects the pumping laser beam 13. The other face 10b ofthe KTP crystal 10 is provided with a coating 125, which substantiallytransmits the laser beam 11 and which substantially reflects the secondharmonic 12.

Therefore, the laser beam 11 having the wavelength of 1,064 nm isconfined between the face 16a and the face 10a, and laser oscillation isthereby caused to occur. The laser beam 11 impinges upon the KTP crystal10, and is converted thereby into the second harmonic 12 having thewavelength of 532 nm. Because the face 10a of the KTP crystal 10 isprovided with the coating 123, the second harmonic 12 is efficientlyradiated out of the KTP crystal 10.

In this embodiment, the KTP crystal 10 is incorporated in a stressapplication means 140. The stress application means 140 comprises asquare frame-like support block 141, and a piezo-electric device 142,which is located inside of the space defined by the support block 141.The stress application means 140 also comprises a drive circuit 143,which applies a voltage E to the piezo-electric device 142 and imparts astrain in the vertical direction in FIG. 12 to the piezo-electric device142. The value of the voltage E applied by the drive circuit 143 can bevaried such that the amount of the strain can be changed over apredetermined range.

The KTP crystal 10 is located in the same orientation as that in thethird embodiment with respect to the laser beam 11, which serves as thefundamental wave. Also, the KTP crystal 10 is accommodated inside of thespace defined by the support block 141 such that the Z axis may extendvertically (i.e., in the direction of the strain of the piezo-electricdevice 142). The lower face of the KTP crystal 10 stands facing thebottom surface of the support block 141 via the heat sink 120 and the TEcooler 121, and the upper face of the KTP crystal 10 is in contact withthe piezo-electric device 142.

In this state, the voltage E is applied to the piezo-electric device142, and the piezo-electric device 142 is strained such that itsthickness (i.e., its vertical dimension) increases. As a result, astress in the Z axis direction is applied to the KTP crystal 10. As thevoltage E is set at a larger value and a larger strain occurs in thepiezo-electric device 142, a larger stress is applied to the KTP crystal10. When the stress applied to the KTP crystal 10 is increased little bylittle to a predetermined value, the phase difference Δ, which has beencaused to occur by the KTP crystal 10, becomes eliminated. Therefore, asin the third embodiment, the second harmonic 12 having the maximumoutput power can be obtained.

In the third and fourth embodiments described above, the fundamentalwave, which has been polarized linearly, impinges upon the KTP crystal10. However, the optical wavelength converting apparatus in accordancewith the present invention, wherein the stress application means isemployed, is also applicable when an unpolarized fundamental wave isconverted into its second harmonic. Also, in such cases, the sameeffects as those described above can be obtained with the opticalwavelength converting apparatus in accordance with the presentinvention. The optical wavelength converting apparatus in accordancewith the present invention, wherein the stress application means isemployed, is additionally applicable when crystals of a nonlinearoptical material other than KTP are employed.

A fifth embodiment of the optical wavelength converting apparatus inaccordance with the present invention will be described hereinbelow withreference to FIG. 13.

By way of example, this embodiment is incorporated in a laser diodepumped solid laser. The laser diode pumped solid laser comprises thephased array laser 14, which produces the laser beam 13 serving as apumping beam, and a collimator lens 215a, which collimates the divergentlaser beam 13. The laser diode pumped solid laser also comprises acondensing lens 215b for condensing the laser beam 13, which has passedthrough the collimator lens 215a. The laser diode pumped solid laseradditionally comprises the Nd:YVO₄ rod 16 and resonator mirrors 217 and218, which are respectively located on the side downwards from theNd:YVO₄ rod 16, i.e. on the right side of the Nd:YVO₄ rod 16 in FIG. 13,and on the side upwards from the Nd:YVO₄ rod 16. The laser diode pumpedsolid laser further comprises the KTP crystal 10, which is locatedbetween the Nd:YVO₄ rod 16 and the resonator mirror 217. These elementsare mounted together on a common case (not shown).

A face 218a of the resonator mirror 218, which face stands facing theNd:YVO₄ rod 16, takes on the form of part of a spherical surface and isprovided with a coating 220, which is of the same type as the coating 18in the embodiment of FIG. 2. A face 217a of the resonator mirror 217,which face stands facing the KTP crystal 10, takes on the form of partof a spherical surface and is provided with a coating 219. The coating219 substantially reflects the laser beam 13 having the wavelength of809 nm, transmits part of the laser beam 11 having the wavelength of1,064 nm, and substantially transmits the second harmonic 12 having thewavelength of 532 nm. Therefore, the laser beam 11 having the wavelengthof 1,064 nm is confined between the face 218a and the face 217a, andlaser oscillation is thereby caused to occur.

The laser beam 11 impinges upon the KTP crystal 10 of a nonlinearoptical material, and is converted thereby into the second harmonic 12having a wavelength, which is one half of the wavelength of the laserbeam 11, i.e. is equal to 532 nm. Because the face 217a of the resonatormirror 217 is provided with the coating 219, the laser beam 11 and thesecond harmonic 12 are radiated out of the resonator mirror 217.

In this embodiment, the KTP crystal 10, which is a biaxial crystal, hasbeen cut along the plane which has been rotated 24° from tile YZ planearound the Z axis (refer to FIG. 5). With this configuration, in caseswhere the KTP crystal 10 is located such that the direction of linearpolarization of the laser beam 11, which direction is indicated by thedouble headed arrow P in FIG. 5, may make an angle of 45° with respectto the Z axis, a large nonlinear optical constant d24 can be utilized,and the type II of phase matching between the laser beam 11, whichserves as the fundamental wave, and its second harmonic 12 can beeffected. As a result, the second harmonic 12 having the maximumintensity can be obtained.

However, if the phase difference Δ is caused to occur in the laser beam11 by the KTP crystal 10, the direction of linear polarization of thelaser beam 11 will change in accordance with the value of the phasedifference Δ. Therefore, it will often occur that the direction oflinear polarization of the laser beam 11, which direction is indicatedby the double headed arrow P, does not make an angle of 45° with respectto the Z axis. How the angle of 45° is achieved between the direction oflinear polarization of the laser beam 11 and the Z axis will bedescribed hereinbelow.

The temperature of the KTP crystal 10 is adjusted by a Peltier device230. The Peltier device 230, which serves as the means for adjusting adifference in phase, is controlled by a Peltier device controller 231.Also, a dichroic mirror 232 is located on the side downwards from theresonator mirror 217. The dichroic mirror 232 transmits the secondharmonic 12 and reflects the laser beam 11, which has been slightlyradiated out of the resonator mirror 217. The laser beam 11, which hasbeen reflected by the dichroic mirror 232, passes through a filter 233for filtering out the second harmonic 12 and then impinges upon apolarization beam splitter 234. The light component of the laser beam11, which component has been polarized linearly in the Y' axisdirection, is reflected by a film surface 234a of the polarization beamsplitter 234 and detected by a first photodetector 235. The lightcomponent of the laser beam 11, which component has been polarizedlinearly in the direction, which is normal to the Y' axis direction,i.e. in the Z axis direction, passes through the film surface 234a ofthe polarization beam splitter 234 and is detected by a secondphotodetector 236.

The photodetectors 235 and 236 may be constituted of photodiodes, or thelike. The photodetectors 235 and 236 detect the light intensities of thetwo linearly polarized light components of the laser beam 11, andgenerate signals S1 and S2. The signals S1 and S2 are fed into adifferential amplifier 237. The differential amplifier 237 generates asignal S3, which has a polarity corresponding to the relationshipbetween the values of the signals S1 and S2. The signal S3 is fed intothe Peltier device controller 231. The Peltier device controller 231controls the Peltier device 230 in accordance with the polarity of thesignal S3 such that the temperature of the KTP crystal 10 may beincreased or decreased. When the Peltier device 230 is operated and thetemperature of the KTP crystal 10 increases or decreases, the phasedifference Δ of the laser beam 11 in the KTP crystal 10 changes. As aresult, the direction of linear polarization of the laser beam 11changes. In this manner, the direction of linear polarization of thelaser beam 11 is controlled such that the light intensity of thepolarized light component, which is detected by the first photodetector235, and the light intensity of the polarized light component, which isdetected by the second photodetector 236, may become equal to eachother. The direction of linear polarization of the laser beam 11, whichhas thus been controlled, makes an angle of 45° with respect to the Zaxis of the KTP crystal 10. Therefore, the wavelength conversionefficiency becomes largest, and the second harmonic 12, having thelargest intensity, can be obtained.

A sixth embodiment of the optical wavelength converting device inaccordance with the present invention will be described hereinbelow withreference to FIG. 14.

In this embodiment, the light input face 16a of the Nd:YVO₄ rod 16 isprovided with the coating 220. A resonator is constituted of the Nd:YVO₄rod 16 and the mirror 217. Also, the light passage face 10a of the KTPcrystal 10 is slant, and the KTP crystal 10 is secured to apiezo-actuator 240. The piezo-actuator 240 is controlled by apiezo-actuator controller 241 and moves the KTP crystal 10 in thevertical direction in FIG. 14.

A dichroic mirror 242 is located on the side downwards from theresonator mirror 217. The dichroic mirror 242 reflects the secondharmonic 12 and transmits the laser beam 11, which has been slightlyradiated out of the resonator mirror 217. The laser beam 11, which haspassed through the dichroic mirror 242, impinges upon the polarizationbeam splitter 234. Therefore, in this embodiment, as in the fifthembodiment, the light component of the laser beam 11, which componenthas been polarized linearly in the Y' axis direction, is detected by thefirst photodetector 235. The light component of the laser beam 11, whichcomponent has been polarized linearly in the Z axis direction, isdetected by the second photodetector 236.

The photodetectors 235 and 236 detect the light intensities of the twolinearly polarized light components of the laser beam 11, and generatethe signals S1 and S2. The signals S1 and S2 are fed into thedifferential amplifier 237. The differential amplifier 237 generates asignal S3, which has a polarity corresponding to the relationshipbetween the values of the signals S1 and S2. The signal S3 is fed intothe piezo-actuator controller 241. The piezo-actuator controller 241controls the piezo-actuator 240 in accordance with the polarity of thesignal S3 such that the KTP crystal 10 may be moved up or down. When thepiezo-actuator 240 is operated and the KTP crystal 10 is moved up ordown, the passage length of the laser beam 11 through the KTP crystal 10changes, and the phase difference Δ of the laser beam 11 changes inaccordance with the change in the passage length of the laser beam 11through the KTP crystal 10. As a result, the direction of linearpolarization of the laser beam 11 changes.

In this manner, the direction of linear polarization of the laser beam11 is controlled such that the light intensity of the polarized lightcomponent, which is detected by the first photodetector 235, and thelight intensity of the polarized light component, which is detected bythe second photodetector 236, may become equal to each other. Thedirection of linear polarization of the laser beam 11, which has thusbeen controlled, makes an angle of 45° with respect to the Z axis of theKTP crystal 10. Therefore, the wavelength conversion efficiency becomesgreatest, and the second harmonic 12, having the greatest intensity canbe obtained.

As the means for adjusting a difference in phase of the laser beam 11,means other than the Peltier device 230 and the piezo-actuator 240 maybe employed. By way of example, the piezo-electric device 250 shown inFIG. 15 may be employed as the means for adjusting a difference inphase. The piezo-electric device 250 is accommodated together with theKTP crystal 10 in a support block 251 such that the piezo-electricdevice 250 may be in close contact with the KTP crystal 10. When avoltage is applied to the piezo-electric device 250, a stress is givenby the piezo-electric device 250 to the KTP crystal 10. As a result, thephase difference Δ in the fundamental wave, which passes through the KTPcrystal 10, changes in accordance with the value of the stress, i.e., inaccordance with the value of the voltage applied to the piezo-electricdevice 250.

A different example of the means for adjusting a difference in phase isillustrated in FIG. 16. The means for adjusting a difference in phaseshown in FIG. 16 comprises two electrodes 260 and 261, which standfacing each other with the KTP crystal 10 intervening therebetween, andan electric power source 262, which applies variable voltage across theelectrodes 260 and 261. When a voltage is applied across the KTP crystal10, the phase difference Δ in the fundamental wave, which passes throughthe KTP crystal 10, changes in accordance with the value of the voltageapplied across the KTP crystal 10.

Into which two polarized light components the fundamental wave isseparated, and what relationship between the light intensities of thetwo polarized light components is investigated are not limited to thosein the fifth and sixth embodiments. For example, the fifth and sixthembodiments may be modified such that the laser beam 11 is separatedinto the polarized light component in the direction making an angle of+45°, with respect to the Z axis of the KTP crystal 10, and thepolarized light component in the direction making an angle of -45°, withrespect to the Z axis of the KTP crystal 10. In such cases, the meansfor adjusting a difference in phase is controlled such that one of thetwo polarized light components may become zero or the other polarizedlight component may take the maximum value.

A seventh embodiment of the optical wavelength converting apparatus inaccordance with the present invention will be described hereinbelow withreference to FIG. 17.

By way of example, this embodiment is incorporated in a laser diodepumped solid laser. The laser diode pumped solid laser comprises thephased array laser 14, which produces the laser beam 13 serving as apumping beam, the collimator lens 215a, and the condensing lens 215b.The laser diode pumped solid laser also comprises the Nd:YVO₄ rod 16 anda resonator mirror 317, which is located on the side downswards from theNd:YVO₄ rod 16, i.e. on the right side of the Nd:YVO₄ rod 16 in FIG. 17.The laser diode pumped solid laser additionally comprises the KTPcrystal 10, which is located between the Nd:YVO₄ rod 16 and theresonator mirror 317. These elements are mounted together on a commoncase (not shown). The neodymium atoms contained in the Nd:YVO₄ rod 16are stimulated by the laser beam 13, and the Nd:YVO₄ rod 16 produces thelinearly polarized laser beam 11 having a wavelength λ2 of 1,064 nm.

The light input face 16a of the Nd:YVO₄ rod 16 is provided with acoating 320, which is of the same type as the coating 18 in theembodiment of FIG. 2. A face 317a of the resonator mirror 317, whichface stands facing the KTP crystal 10, takes on the form of part of aspherical surface and is provided with a coating 319, which is of thesame type as the coating 19 in the embodiment of FIG. 2. Therefore, thelaser beam 11 having a wavelength of 1,064 nm is confined between theface 16a and the face 317a, and laser oscillation is thereby caused tooccur.

The laser beam 11 impinges upon the KTP crystal 10 of a nonlinearoptical material, and is converted thereby into the second harmonic 12having a wavelength, which is one half of the wavelength of the laserbeam 11, i.e. is equal to 532 nm. Because the face 317a of the resonatormirror 317 is provided with the coating 319, approximately only thesecond harmonic 12 is radiated out of the resonator mirror 317.

As illustrated in detail in FIG. 18, the KTP crystal 10, which is abiaxial crystal, is located such that the X axis may make an angle ofφ=24°, with respect to the direction of incidence of the laser beam 11,which serves as the fundamental wave, and the Z axis may make an angleof θ=90°, with respect to the direction of incidence of the laser beam11. With this configuration, in cases where the KTP crystal 10 islocated such that the direction of linear polarization of the laser beam11, which direction is indicated by the double headed arrow P, may makean angle of 45°, with respect to the Z axis, a large nonlinear opticalconstant d24 can be utilized, and the type II of phase matching betweenthe laser beam 11, which serves as the fundamental wave, and its secondharmonic 12 can be effected. As a result, the second harmonic 12, havingthe maximum intensity, can be obtained.

However, if the phase difference Δ is caused to occur in the laser beam11 by the KTP crystal 10, the direction of linear polarization of thelaser beam 11 will change in accordance with the value of the phasedifference Δ. Therefore, it will often occur that the direction oflinear polarization of the laser beam 11, which direction is indicatedby the double headed arrow P, does not make an angle of 45° with respectto the Z axis. How the angle of 45° is achieved between the direction oflinear polarization of the laser beam 11 and the Z axis will bedescribed hereinbelow.

The KTP crystal 10 is secured to a rotation shaft 330, which extendsparallel to the Z axis. The rotation shaft 330 is supported on a supportbase 331 such that the rotation shaft 330 can rotate. An adjusting knob333, which can rotate around a rotation shaft 332, is engaged with thesupport base 331. The rotation shaft 332 is coupled with the rotationshaft 330 via reduction gears. Therefore, when the adjusting knob 333 isrotated, the KTP crystal 10 can be rotated around the rotation shaft330.

When the KTP crystal 10 is rotated in the manner described above, thelength L of the optical path of the laser beam 11 in the KTP crystal 10changes. If the length L of the optical path thus changes, the phasedifference Δ, which occurs in the laser beam 11 by the KTP crystal 10,changes. As a result, the direction of linear polarization of the laserbeam 11 changes. Accordingly, by rotating the KTP crystal 10 little bylittle, the angle of 45° can be achieved between the direction of linearpolarization of the laser beam 11 and the Z axis, and the secondharmonic 12, having the maximum intensity, can be obtained.

In this embodiment, the KTP crystal 10 is rotated in the direction suchthat the angle φ may change. Alternatively, the KTP crystal 10 may berotated in a direction such that the angle θ may change or such thatboth the angles φ and θ may change. The KTP crystal 10 should preferablybe rotated in a direction such that both the angles φ and θ may change.In such cases, the possible range of adjustment of the phase differenceΔ becomes wide.

Also, it is necessary for the amount of rotation of the KTP crystal 10to be adjusted such that the angles φ and θ may take values in which thephase matching between the laser beam 11 and its second harmonic 12 canbe effected. In cases where the standard angles are θ=90° and φ=24°, theallowable angle ranges Δθ and Δφ for the phase matching are representedby the formulas

    L.sup.1/2 ·Δθ=60 mrad·cm.sup.1/2

    L·Δφ=17 mrad·cm

where L is in units of cm. Therefore, the angles θ and φ should beadjusted such that the conditions

    |Δθ|≦60/L.sup.2/1  (mrad)

    |Δφ|≦17/L (mrad)

may be satisfied. Specifically, in cases where the KTP crystal 10 hasthe standard length of L=0.5 cm, the angles θ and φ may be adjusted suchthat the conditions

    |Δθ|≦85 mrad

    |Δφ|≦34 mrad

may be satisfied.

In this embodiment, the laser beam 11 impinges from the obliquedirection upon the light input face 10a of the KTP crystal 10. Also, thelaser beam 11 and its second harmonic 12 are radiated in an obliquedirection out of the light output face 10b. Therefore, the phasedifference Δ can be changed more with a smaller angle of rotation of theKTP crystal 10 than when the laser beam 11 impinges perpendicularly uponthe light input face 10a of the KTP crystal 10 and the laser beam 11,and its second harmonic 12, are radiated perpendicularly out of thelight output face 10b.

An eighth embodiment of the optical wavelength converting device inaccordance with the present invention will be described hereinbelow withreference to FIG. 19.

This embodiment is provided with two KTP crystals 10, 10. As in theseventh embodiment, the two KTP crystals 10, 10 are rotated aroundrotation shafts 330, 330 by rotation mechanisms, which are coupled withthe adjusting knob 333. The rotation mechanisms rotate the KTP crystals10, 10 such that they may be symmetric with respect to a plane H, whichextends in the direction normal to the optical axes of the lenses 215aand 215b, and at the middle between the rotation shafts 330, 330.

In cases where the KTP crystals 10, 10 are rotated in the mannerdescribed above, the direction of the optical path of the secondharmonic 12, which has been radiated out of the right KTP crystal 10, iskept constant regardless of the angles of rotation of the KTP crystals10, 10. Therefore, the second harmonic 12 can impinge, at apredetermined angle of incidence, upon an apparatus, which utilizes thesecond harmonic 12.

A ninth embodiment of the optical wavelength converting apparatus inaccordance with the present invention will be described hereinbelow withreference to FIG. 20.

By way of example, this embodiment is incorporated in a laser diodepumped solid laser. The laser diode pumped solid laser comprises thephased array laser 14, which produces the laser beam 13 serving as apumping beam, the collimator lens 215a, and the condensing lens 215b.The laser diode pumped solid laser also comprises Nd:YVO₄ rods 16A and16B and a resonator mirror 417, which is located on the side downwardsfrom the Nd:YVO₄ rods 16A and 16B, i.e. on the right side of the Nd:YVO₄rods 16A and 16B in FIG. 20. The laser diode pumped solid laseradditionally comprises the KTP crystal 10, which is located between theNd:YVO₄ rods 16A, 16B and the resonator mirror 417. These elements aremounted together on a common case (not shown). The neodymium atomscontained in the Nd:YVO₄ rods 16A and 16B are stimulated by the laserbeam 13, and the Nd:YVO₄ rods 16A and 16B produce the linearly polarizedlaser beam 11 having a wavelength of 1,064 nm.

The light input face 16a of the Nd:YVO₄ rod 16A is provided with acoating 420, which is of the same type as the coating 18 in theembodiment of FIG. 2. A face 417a of the resonator mirror 417, whichface stands facing the KTP crystal 10, takes on the form of part of aspherical surface and is provided with a coating 419, which is of thesame type as the coating 19 in the embodiment of FIG. 2. Therefore, thelaser beam 11 having the wavelength of 1,064 nm is confined between theface 16a and the face 417a, and laser oscillation is thereby occurs.

As illustrated in detail in FIG. 21, the KTP crystal 10, which is abiaxial crystal having the nonlinear optical effects, has been cut alongthe plane which has been rotated 24° from the YZ plane around the Zaxis. In this embodiment, a large nonlinear optical constant d24 of theKTP crystal 10 is utilized. Therefore, a fundamental wave, which has thelinearly polarized light component in the Z axis direction and thelinearly polarized light component in the Y' axis direction, must becaused to impinge upon the KTP crystal 10. For this purpose, the Nd:YVO₄rod 16A is located in an orientation such that it may produce the lightcomponent of the laser beam 11, which component has been polarizedlinearly in the Z axis direction. Also, the Nd:YVO₄ rod 16B is locatedin an orientation such that it may produce the light component of thelaser beam 11, which component has been polarized linearly in the Y'axis direction.

The laser beam 11, which is composed of the two linearly polarized lightcomponents produced by the Nd:YVO₄ rods 16A and 16B, impinges upon theKTP crystal 10, and is converted thereby into the second harmonic 12having a wavelength, which is one half of the wavelength of the laserbeam 11, i.e. is equal to 532 nm. At this time, the type II of phasematching between the laser beam 11, which serves as the fundamentalwave, and its second harmonic 12 can be effected. As a result, thesecond harmonic 12 having the maximum intensity can be obtained. Becausethe face 417a of the resonator mirror 417 is provided with the coating419, approximately only the second harmonic 12 is radiated out of theresonator mirror 417.

Of the linearly polarized light components of the laser beam 11impinging upon the KTP crystal 10, the linearly polarized lightcomponent, which has been produced by the Nd:YVO₄ rod 16A, is subjectedonly to the refractive index represented by Formula (10) and is notsubjected to the refractive index represented by Formula (11). Also, thelinearly polarized light component of the laser beam 11, which componenthas been produced by the Nd:YVO₄ rod 16B, is subjected only to therefractive index represented by Formula (11) and is not subjected to therefractive index represented by Formula (10). Therefore, it does notoccur that each of the two linearly polarized light components of thelaser beam 11 is subjected to both the two refractive indexes.Accordingly, with this embodiment, even if the phase difference Δrepresented by Formula (12) fluctuates, the direction of linearpolarization of the laser beam 11 does not change, and the secondharmonic 12, having a large intensity, can be obtained reliably.

A tenth embodiment of the optical wavelength converting device inaccordance with the present invention will be described hereinbelow withreference to FIG. 22.

In this embodiment, the coating 420 is overlaid on the light input face16a of the Nd:YVO₄ rod 16A, and the coating 419 is overlaid on the rightface 10a of the KTP crystal 10. A resonator is constituted of theNd:YVO₄ rod 16A and the KTP crystal 10. The phased array laser 14, whichserves as the pumping source, is located close to the Nd:YVO₄ rod 16A.

In this embodiment, as in the ninth embodiment, the Nd:YVO₄ rod 16A islocated in an orientation such that it may produce the light componentof the laser beam 11, which component has been polarized linearly in theZ axis direction. Also, the Nd:YVO₄ rod 16B is located in an orientationsuch that it may produce the light component of the laser beam 11, whichcomponent has been polarized linearly in the Y' axis direction. In thismanner, with this embodiment, a large nonlinear optical constant d24 ofthe KTP crystal 10 can be utilized, and any change in the direction oflinear polarization of the laser beam 11 can be prevented from occurringeven if the phase difference Δ fluctuates.

The two solid laser media employed in combination with each other in theninth and tenth embodiments are not limited to Nd:YVO₄, and may beselected from any other media, e.g. a direct compound laser crystal,such as LNP, which produce a linearly polarized laser beam.

An eleventh embodiment of the optical wavelength converting apparatus inaccordance with the present invention will be described hereinbelow withreference to FIG. 23.

A laser diode pumped solid laser, which is provided with this embodimentof the optical wavelength converting device in accordance with thepresent invention, comprises the phased array laser 14, which producesthe laser beam 13 serving as a pumping beam, the collimator lens 215a,and the condensing lens 215b. The laser diode pumped solid laser alsocomprises the Nd:YVO₄ rod 16 and a resonator mirror 517, which islocated on the side downwards from the Nd:YVO₄ rod 16, i.e. on the rightside of the Nd:YVO₄ rod 16 in FIG. 23. The laser diode pumped solidlaser additionally comprises the KTP crystal 10, which is locatedbetween the Nd:YVO₄ rod 16 and the resonator mirror 517. These elementsare mounted together on a common case (not shown). The neodymium atomscontained in the Nd:YVO₄ rod 16 are stimulated by the laser beam 13, andthe Nd:YVO₄ rod 16 produces the laser beam 11 having a wavelength of1,064 nm.

The light input face 16a of the Nd:YVO₄ rod 16 is provided with acoating 518, which is of the same type as the coating 18 in theembodiment of FIG. 2. Also, the light output face 16b of the Nd:YVO₄ rod16 is provided with a non-reflective coating 509, which substantiallytransmits the laser beam 11 having a wavelength of 1,64 nm (with atransmittance of at least 99.9%). A face 517a of the resonator mirror517, which face stands facing the KTP crystal 10, takes on the form ofpart of a spherical surface and is provided with a coating 519, which isof the same type as the coating 19 in the embodiment of FIG. 2.Therefore, the laser beam 11, having a wavelength of 1,064 nm, isconfined between the face 16a and the face 517a, and laser oscillationthereby occurs. Though not shown, the face of the KTP crystal 10, whichface stands facing the Nd:YVO₄ rod 16, is provided with a coating, whichis of the same type as the non-reflective coating 509.

The laser beam 11 impinges upon the KTP crystal 10 of a nonlinearoptical material, and is converted thereby into the second harmonic 12having a wavelength, which is one half of the wavelength of the laserbeam 11, i.e. is equal to 532 nm. Because the face 517a of the resonatormirror 517 is provided with the coating 519, approximately only thesecond harmonic 12 is radiated out of the resonator mirror 517.

The Nd:YVO₄ rod 16, which exhibits birefringence, is shaped in awedge-like form such that the thickness of the Nd:YVO₄ rod 16 changeslittle by little along the direction intersecting the optical path ofthe laser beam 11 in the Nd:YVO₄ rod 16, i.e. in the vertical directionin FIG. 23. The Nd:YVO₄ rod 16 is supported on a support member 521. Aplurality of guide rods 522, 522, . . . , which extend vertically inFIG. 23, are inserted through the support member 521. The lower ends ofthe guide rods 522, 522, . . . are secured to a fixing base 523. Thesupport member 521 can move vertically along the guide rods 522, 522, .. . A precision screw 524 is supported by the fixing base 523 such thatit can move. The leading end of the precision screw 524 is engaged bythreads with the support member 521. Therefore, when the precision screw524 is rotated, the support member 521 is moved thereby in the verticaldirection. As a result, the Nd:YVO₄ rod 16 is moved vertically.

As illustrated in detail in FIG. 24, the KTP crystal 10, which is abiaxial crystal, has been cut along the plane which has been rotated 24°from the YZ plane around the Z axis. With this configuration, in caseswhere the KTP crystal 10 is located such that the direction of linearpolarization of the laser beam 11, which direction is indicated by thedouble headed arrow P, may make an angle of 45° with respect to the Zaxis, a large nonlinear optical constant d24 can be utilized, and thetype II of phase matching between the laser beam 11, which serves as thefundamental wave, and its second harmonic 12 can be effected. As aresult, the second harmonic 12, having the maximum intensity, can beobtained.

However, if the phase difference Δ is caused to occur in the laser beam11 by the KTP crystal 10, the direction of linear polarization of thelaser beam 11 will change in accordance with the value of the phasedifference Δ. Therefore, it will often occur that the direction oflinear polarization of the laser beam 11, which direction is indicatedby the double headed arrow P, does not make an angle of 45° with respectto the Z axis. With this embodiment, the precision screw 524 is rotatedclockwise or counter-clockwise, and the Nd:YVO₄ rod 16 is thereby movedlittle by little in the vertical direction. As a result, the length ofthe optical path of the laser beam 11 in the Nd:YVO₄ rod 16 changes, andthe phase difference Δ' described above changes. Therefore, thedirection of linear polarization of the laser beam 11 changes. Byadjusting the direction of linear polarization of the laser beam 11 inthis manner, the angle of the direction of linear polarization of thelaser beam 11, with respect to the Z, axis can be set at 45°. In thisstate, the second harmonic 12, having the maximum intensity, can beobtained.

The amount of birefringence of Nd:YVO₄ is n_(e) -n_(o) =0.2079, which ismarkedly larger than the amount of birefringence of KTP, n_(Z) -n_(Y)=0.0853. Therefore, in this embodiment, the inclination of the face 16bof the Nd:YVO₄ rod 16 may be gentler, and the distance of the movementof the Nd:YVO₄ rod 16 may be smaller than when the face of the KTPcrystal 10 is formed as a slant surface and the KTP crystal 10 is moved.Accordingly, with this embodiment, the direction of linear polarizationof the laser beam 11 can be set appropriately with a small amount ofadjustment such that the position of the resonator mode may not shift.

A twelfth embodiment of the optical wavelength converting device inaccordance with the present invention will be described hereinbelow withreference to FIG. 25.

In the twelfth embodiment, the phased array laser 14 is located close tothe Nd:YVO₄ rod 16. The laser beam 13, which has been produced by thephased array laser 14, directly impinges upon the Nd:YVO₄ rod 16. Thecoating 519, which is of the same type as the coating 519 in theembodiment of FIG. 23, is overlaid on the right face 10b of the KTPcrystal 10. A resonator is constituted of the KTP crystal 10 and theNd:YVO₄ rod 16. The light input face 10a of the KTP crystal 10 is cutobliquely, and the wedge-like Nd:YVO₄ rod 16 is adhered to the face 10a.The KTP crystal 10 is supported on the support member 521, which is ofthe same type as the support member 521 in the embodiment of FIG. 23.When the precision screw 524 is rotated, the KTP crystal 10 is movedtogether with the Nd:YVO₄ rod 16 in the vertical direction in FIG. 25.With this embodiment, by vertically moving the Nd:YVO₄ rod 16, thedirection of linear polarization of the laser beam 11 can be adjusted.

A thirteenth embodiment of the optical wavelength converting device inaccordance with the present invention will be described hereinbelow withreference to FIG. 26.

In the thirteenth embodiment, the structure of the resonator on thesecond harmonic radiating side differs from that in the embodiment ofFIG. 23. Specifically, in this embodiment, the right face 10b of the KTPcrystal 10 takes on the form of part of a spherical surface and isprovided with the coating 519, which is of the same type as the coating519 in the embodiment of FIG. 23. With the thirteenth embodiment, thenumber of parts can be reduced by one as compared with the embodiment ofFIG. 23. Therefore, the thirteenth embodiment is advantageous in keepingthe optical wavelength converting device small in size, light in weight,and cheap in cost.

In the eleventh, twelfth, and thirteenth embodiments described above,the phased array laser 14, which serves as the pumping source, is keptstationary, and the Nd:YVO₄ rod 16 is moved. Alternatively, the Nd:YVO₄rod 16 may be kept stationary, and the phased array laser 14 may bemoved (together with the lenses 215a and 215b in the embodiments ofFIGS. 23 and 26).

A fourteenth embodiment of the optical wavelength converting apparatusin accordance with the present invention will be described hereinbelowwith reference to FIG. 24.

A laser diode pumped solid laser, which is provided with this embodimentof the optical wavelength converting device in accordance with thepresent invention, comprises the phased array laser 14, which producesthe laser beam 13 serving as a pumping beam, the collimator lens 215a,and the condensing lens 215b. The laser diode pumped solid laser alsocomprises the Nd:YVO₄ rod 16 and a resonator mirror 617, which islocated on the side downwards from the Nd:YVO₄ rod 16, i.e. on the rightside of the Nd:YVO₄ rod 16 in FIG. 27. The laser diode pumped solidlaser additionally comprises the KTP crystal 10, which is locatedbetween the Nd:YVO₄ rod 16 and the resonator mirror 617. The laser diodepumped solid laser further comprises an etalon plate 620, which islocated between the Nd:YVO₄ rod 16 and the KTP crystal 10. Theseelements are mounted together on a common case (not shown). Theneodymium atoms contained in the Nd:YVO₄ rod 16 are stimulated by thelaser beam 13, and the Nd:YVO₄ rod 16 produces the laser beam 11 havinga fundamental wavelength of 1,064 nm.

The light input face 16a of the Nd:YVO₄ rod 16 is provided with acoating 618, which is of the same type as the coating 18 in theembodiment of FIG. 2. Also, the light output face 16b of the Nd:YVO₄ rod16 is provided with a non-reflective coating 609, which substantiallytransmits the laser beam 11 having a wavelength of 1,64 nm (with atransmittance of at least 99.9%). A face 617a of the resonator mirror617, which face stands facing the KTP crystal 10, takes on the form ofpart of a spherical surface and is provided with a coating 619, which isof the same type as the coating 19 in the embodiment of FIG. 2.Therefore, the laser beam 11 having a wavelength of 1,064 nm is confinedbetween the face 16a and the face 617a, and laser oscillation therebyoccurs. Both faces of the etalon plate 620 are not provided with anon-reflective coating. Though not shown, the face of the KTP crystal10, which face stands facing the etalon plate 620, is provided with acoating, which is of the same type as the non-reflective coating 609.

The laser beam 11 impinges upon the KTP crystal 10 of a nonlinearoptical material, and is converted thereby into the second harmonic 12having a wavelength, which is one half of the wavelength of the laserbeam 11, i.e. is equal to 532 nm. Because the face 617a of the resonatormirror 617 is provided with the coating 619, approximately only thesecond harmonic 12 is radiated out of the resonator mirror 617.

The etalon plate 620, which serves as a wavelength selecting device, isshaped in a wedge-like form. The etalon plate 620 is supported on asupport member 621. A plurality of guide rods 622, 622, . . . , whichextend vertically in FIG. 27, are inserted through the support member621. The lower ends of the guide rods 622, 622, . . . are secured to afixing base 623. The support member 621 can move vertically along theguide rods 622, 622, . . . A precision screw 624 is supported by thefixing base 623 such that it can move. The leading end of the precisionscrew 624 is engaged by threads with the support member 621. Therefore,when the precision screw 624 is rotated, the support member 621 is movedthereby in the vertical direction. As a result, the etalon plate 620 ismoved vertically. As the etalon plate 620 is inserted into the opticalpath of the laser beam 11, the wavelength of the laser beam 11 isselectively set at a predetermined value in accordance with thethickness of the etalon plate 620.

The KTP crystal 10, which is a biaxial crystal, has been cut along theplane which has been rotated 24° from the YZ plane around the Z axis(refer to FIG. 5). With this configuration, in cases where the KTPcrystal 10 is located such that the direction of linear polarization ofthe laser beam 11, which direction is indicated by the double headedarrow P, may make an angle of 45° with respect to the Z axis, a largenonlinear optical constant d24 can be utilized, and the type II of phasematching between the laser beam 11, which serves as the fundamentalwave, and its second harmonic 12 can be effected. As a result, thesecond harmonic 12 having the maximum intensity can be obtained.

However, if the phase difference Δ is caused to occur in the laser beam11 by the KTP crystal 10, the direction of linear polarization of thelaser beam 11 will change in accordance with the value of the phasedifference Δ. Therefore, it will often occur that the direction oflinear polarization of the laser beam 11, which direction is indicatedby the double headed arrow P, does not make an angle of 45° with respectto the Z axis. With this embodiment, the precision screw 624 is rotatedclockwise or counter-clockwise, and the etalon plate 620 is therebymoved little by little in the vertical direction. As a result, thelength of the optical path of the laser beam 11 in the etalon plate 620changes, and the selected value of the wavelength changes little bylittle. When the wavelength of the laser beam 11 changes, the phasedifference Δ described above changes. Therefore, the direction of linearpolarization of the laser beam 11 changes. By adjusting the direction oflinear polarization of the laser beam 11 in this manner, the angle ofthe direction of linear polarization of the laser beam 11, with respectto the Z axis, can be set at 45°. In this state, the second harmonic 12,having the maximum intensity, can be obtained. The wedge-like etalonplate 620 may be formed such that one light passage face may make anangle of, for example, approximately 1' with respect to the other lightpassage face.

In cases where the etalon plate 620 is employed, the laser diode pumpedsolid laser oscillates in a single longitudinal mode. Therefore, nolongitudinal mode competition occurs in the laser diode pumped solidlaser. Accordingly, no noise due to longitudinal mode competitionoccurs.

A fifteenth embodiment of the optical wavelength converting device inaccordance with the present invention will be described hereinbelow withreference to FIG. 28.

In the fifteenth embodiment, two light passage faces of an etalon plate630 are formed as parallel flat surfaces. The etalon plate 630 issupported on a support member 631 such that it can rotate around arotation shaft 632. An adjusting knob 633 is engaged with the supportmember 631 such that the adjusting knob 633 can rotate around a rotationshaft 634. The rotation shaft 634 is coupled with the rotation shaft 632via reduction gears (not shown). Therefore, when the adjusting knob 633is rotated, the etalon plate 630 rotates around the rotation shaft 632.When the etalon plate 630 thus rotates, the length of the passage of thelaser beam 11 through the etalon plate 630 changes. Accordingly, in thefifteenth embodiment, as in the fourteenth embodiment, the direction oflinear polarization of the laser beam 11 can be adjusted.

A sixteenth embodiment of the optical wavelength converting device inaccordance with the present invention will be described hereinbelow withreference to FIG. 29.

In the sixteenth embodiment, the Nd:YVO₄ rod 16 is shaped in awedge-like form. The light output face 16b of the Nd:YVO₄ rod 16 is notprovided with the non-reflective coating 609 (shown in FIGS. 27 and 28).With this Nd:YVO₄ rod 16, part (e.g. approximately 20%) of the laserbeam 11 is reflected from the light output face 16b towards the lightinput face 16a, and a standing wave thus occurs. Therefore, the Nd:YVO₄rod 16 also serves as a wavelength selecting device, with which theselected wavelength can be adjusted. The Nd:YVO₄ rod 16 is supported onthe support member 621, which is of the same type as the support member621 in the embodiment of FIG. 27. When the precision screw 624 isrotated, the Nd:YVO₄ rod 16 can be moved vertically in FIG. 29. Withthis embodiment, by moving the Nd:YVO₄ rod 16 up and down, thewavelength of the laser beam 11 can be changed, and the direction oflinear polarization of the laser beam 11 can be adjusted.

A seventeenth embodiment of the optical wavelength converting apparatusin accordance with the present invention will be described hereinbelowwith reference to FIG. 30.

A laser diode pumped solid laser, which is provided with this embodimentof the optical wavelength converting device in accordance with thepresent invention, comprises the phased array laser 14, which producesthe laser beam 13 serving as a pumping beam, the collimator lens 215a,and the condensing lens 215b. The laser diode pumped solid laser alsocomprises the Nd:YVO₄ rod 16 and the KTP crystal 10, which is located onthe side downwards from the Nd:YVO₄ rod 16, i.e. on the right side ofthe Nd:YVO₄ rod 16 in FIG. 30. These elements are mounted together on acommon case (not shown). The neodymium atoms contained in the Nd:YVO₄rod 16 are stimulated by the laser beam 13, and the Nd:YVO₄ rod 16produces the laser beam 11 having a wavelength of 1,064 nm.

The light input face 16a of the Nd:YVO₄ rod 16 is provided with acoating 718, which is of the same type as the coating 18 in theembodiment of FIG. 2. The right face 10a of the KTP crystal 10 isprovided with a coating 719, which is of the same type as the coating 19in the embodiment of FIG. 2. Also, the left face 10b of the KTP crystal10 takes on the form of part of a convex spherical surface and isprovided with a coating 720, which substantially transmits the laserbeam 11. Therefore, the laser beam 11 having a wavelength of 1,064 nm isconfined between the face 16a and the face 10a, and laser oscillationthereby occurs.

The laser beam 11 impinges upon the KTP crystal 10 of a nonlinearoptical material, and is converted thereby into the second harmonic 12having a wavelength, which is one half of the wavelength of the laserbeam 11, i.e. is equal to 532 nm. Because the face 10a of the KTPcrystal 10 is provided with the coating 719, approximately only thesecond harmonic 12 is radiated out of the KTP crystal 10.

The KTP crystal 10 is supported on a support member 721. A plurality ofguide rods 722,722, . . . , which extend vertically in FIG. 30, areinserted through the support member 721. The lower ends of the guiderods 722,722, . . . are secured to a fixing base 723. The support member721 can move vertically along the guide rods 722, 722, . . . A precisionscrew 724 is supported by the fixing base 723 such that it can move. Theleading end of the precision screw 724 is engaged by threads with thesupport member 721. Therefore, when the precision screw 724 is rotated,the support member 721 is moved thereby in the vertical direction. As aresult, the KTP crystal 10 is moved vertically.

As illustrated in detail in FIG. 31, the KTP crystal 10, which is abiaxial crystal, is located such that the laser beam 11 may impingeperpendicularly upon the plane, which has been rotated 24° from the YZplane around the Z axis. With this configuration, in cases where the KTPcrystal 10 is located such that the direction of linear polarization ofthe laser beam 11, which direction is indicated by the double headedarrow P, may make an angle of 45° with respect to the Z axis, a largenonlinear optical constant d24 can be utilized, and the type II of phasematching between the laser beam 11, which serves as the fundamentalwave, and its second harmonic 12 can be effected. As a result, thesecond harmonic 12 having the maximum intensity can be obtained.

However, if the phase difference Δ is caused to occur in the laser beam11 by the KTP crystal 10, the direction of linear polarization of thelaser beam 11 will change in accordance with the value of the phasedifference Δ. Therefore, it will often occur that the direction oflinear polarization of the laser beam 11, which direction is indicatedby the double headed arrow P, does not make an angle of 45° with respectto the Z axis. With this embodiment, the precision screw 724 is rotatedclockwise or counter-clockwise, and the KTP crystal 10 is thereby movedlittle by little in the vertical direction. As a result, the length L ofthe optical path of the laser beam 11 in the KTP crystal 10 changeslittle by little. When the length L of the optical path of the laserbeam 11 in the KTP crystal 10 changes, the phase difference Δ describedabove changes. Therefore, the direction of linear polarization of thelaser beam 11 changes. By adjusting the direction of linear polarizationof the laser beam 11 in this manner, the angle of the direction oflinear polarization of the laser beam 11 with respect to the Z axis canbe set at 45°. In this state, the second harmonic 12 having the maximumintensity can be obtained.

Also, in this embodiment, the face 10b of the KTP crystal 10 takes onthe form of part of a convex spherical surface. Therefore, the laserbeam 11 is easily generated in a single longitudinal mode. Accordingly,mode competition noise can be prevented from occurring, and the secondharmonic 12 free of any noise can be obtained. Additionally, because theface 10b of the KTP crystal 10 has lens effects, the diameter of thelaser beam 11 in the KTP crystal 10 becomes small. Therefore, thewavelength conversion efficiency can be kept high.

An eighteenth embodiment of the optical wavelength converting device inaccordance with the present invention will be described hereinbelow withreference to FIG. 32.

In the eighteenth embodiment, the KTP crystal 10 is supported on asupport member 731 such that the KTP crystal 10 can rotate around arotation shaft 732, which is shifted from the center of the curvature ofthe face 10b. An adjusting knob 733 is engaged with the support member731 such that the adjusting knob 733 can rotate around a rotation shaft734. The rotation shaft 734 is coupled with the rotation shaft 732 viareduction gears. Therefore, when the adjusting knob is rotated, the KTPcrystal 10 rotates around the rotation shaft 732. When the KTP crystal10 thus rotates, the length of the optical path of the laser beam 11 inthe KTP crystal 10 changes. Accordingly, with this embodiment, as in theseventeen embodiment, the direction of linear polarization of the laserbeam 11 can be adjusted.

What is claimed is:
 1. An optical wavelength converting apparatus inwhich a laser beam serving as a fundamental wave and having beenobtained by pumping a solid laser medium impinges upon a crystal of anonlinear optical material, the type II of phase matching between thefundamental wave and its second harmonic is effected, and the secondharmonic of the fundamental wave is thereby radiated out of the opticalwavelength converting apparatus,wherein a solid laser medium, whichexhibits birefringence and the thickness of which changes graduallyalong the direction that intersects the optical path of said fundamentalwave in the solid laser medium, is employed as said solid laser medium,and a means which moves said solid laser medium with respect to apumping source along said direction that intersects the optical path ofsaid fundamental wave.
 2. An apparatus as defined in claim 1 whereinsaid crystal and said solid laser medium are located such that they maybe in close contact with each other.
 3. An apparatus as defined in claim1 wherein said pumping source for pumping said solid laser medium is alaser diode.
 4. An apparatus as defined in claim 1 wherein said crystalis located inside of a resonator of a solid laser.
 5. An apparatus asdefined in claim 1 wherein YVO₄, which has been added with Nd, isemployed as said solid laser medium.
 6. An apparatus as defined in claim1 wherein said crystal of said nonlinear optical material is a KTPcrystal.
 7. An apparatus as defined in claim 6 wherein said KTP crystalhas been cut along a plane, which has been rotated 24° from its YZ planearound the Z axis, and is located in an orientation such that saidfundamental wave impinges perpendicularly upon the cut surface and suchthat the direction of linear polarization of said fundamental wave maymake an angle of 45°, with respect to the Z axis.